Such devices are frequently used for example in the field of dentistry. The device comprises, or is constituted of, a hand part that can be provided for example with a bur, another rotating tool, or a device driven by a rotary motor. The electronic servo-control module is often connected to the patient's chair and connected to the manual instrument by a flexible connection containing electric leads and passage tubes for air and water.
The rotary motors used in this type of hand part are more and more often motors without collectors and without brushes, which have the advantage of being more robust and being capable of being sterilized completely. In order to avoid excessive heating of the instrument, which can be called upon to turn at considerable speeds, for example up to 40'000 turns per minute, three-phase motor are preferable.
Such dental devices are described for example in documents EP688539, U.S. Pat. No. 5,543,695, EP1228737, EP1302173, WO0105023, WO01/45248, EP1109301 or WO0004631, to which the reader will refer advantageously.
It is important for the user of this type of device to control the instrument's rotation speed accurately and over a large range. A torque control is also necessary for example to stop the rotation before the tool breaks. In the prior art, solutions are therefore known in which the manual instruments are provided with sensors making it possible to detect at any time the angular position or the rotor speed of the motor. The motor is servo-controlled with set-point values determined taking into account the measurement signals supplied by these sensors.
In order to increase the flexibility of the connection between the servo-control module and the instrument, it is necessary to limit the number of electric leads going through this connection. The connections that are commercially available and accepted by the users thus comprise two, or at most four electric leads.
In the case of a three-phase motor, three of these leads are used for powering stator coils (phases of the motor). The fourth lead is often used for controlling the lamp or another accessory associated with the hand part. No electric conductor is thus available for transmitting the measurement signals supplied by the sensor to the electronic servo-control module.
Different solutions exist in the prior art in which the servo-controlling electronics are completely loaded in the manual instrument. This type of hand part is however voluminous, heavy and thus difficult to handle. Furthermore, the cost of a set of several instruments that all include servo-control electronics becomes prohibitive.
Other solutions have been suggested in which the servo-control module injects on at least one of the phase conductors a high-frequency signal deformed by the stator and rotor magnetic fields. The measuring of this deformation on the return conductors allows the rotor's angular position to be determined. This method is however not very robust and does not allow reliable and accurate measurements to be performed.
Other servo-control solutions used employ non-standard connections with more than four electric leads.
Other solutions further require the rotor's configuration to be modified, for example its shape or the arrangement of the magnets, to allow a measurement without loaded electronics. These solutions do not apply with standard commercial motors and set important additional constraints during manufacture of the motor.
One aim of the present invention is thus to propose an improved instrument allowing the mentioned problems of the prior art to be resolved.
One aim of the present invention is in particular to propose an improved device in which the rotor's angular position can be determined with an accuracy and a resolution on the order of 1 degree, and this on a very large speed range, for example from 1 to 40'000 turns per minute.
Another aim is to increase the possibilities for control, measure and servo-control of an instrument of this type.